The present invention relates to a palladium (Pd)-coated copper wire for ball bonding, the wire being suitable for connecting an IC chip electrode to an external lead or the like on a substrate in a semiconductor device. More specifically, the present invention relates to an extra fine palladium (Pd)-coated copper wire having a diameter of 10 to 25 μm and capable of giving a stable molten ball.
Generally, a process called ball bonding is employed for the first bonding of a coated copper bonding wire to an electrode, and a process called wedge bonding is employed for the second bonding of the coated copper bonding wire to wiring on a semiconductor circuit wiring board. In the first bonding, a discharge current in an electronic-flame-off (EFO) process is used to apply arc input heat to the tip of the coated copper bonding wire. In the EFO process, in general, an angle formed between the tip of the bonding wire and the tip of a discharge torch is 60 degrees or less with respect to the longitudinal direction of the wire. In the process, arc discharge is formed between the tips of the discharge torch and the wire, a molten ball portion is formed at the tip of the bonding wire and the ball portion is connected to an aluminum pad on an electrode.
The tip portion of the bonding wire is molten to form the ball, and thereafter surface tension is utilized to solidify its molten item. Specifically, at the tip of the molten bonding wire, a sphericity called a free air ball (FAB) is formed. Then, while the initial ball and the electrode are being heated at a temperature of 150 to 300° C., ultrasonic waves are applied, and thus the initial ball and the electrode are crimped and are thereby bonded to the aluminum pad on a chip.
Here, the FAB refers to a molten ball that is formed at the tip of the bonding wire by performing spark discharge on the tip of the bonding wire while spraying a non-oxidizing gas such as nitrogen or nitrogen-hydrogen or a reducing gas to the tip of the coated copper bonding wire which is extended out from the tip of a bonding tool. Examples of the material of the aluminum pad include 99.99% by mass or more of pure aluminum (Al), an alloy of aluminum (Al)-1% by mass of silicon (Si), an alloy of aluminum (Al)-0.5% by mass of copper (Cu) and an alloy of aluminum (Al)-1% by mass of silicon (Si)-0.5% by mass of copper (Cu).
Conventionally, as a bonding wire for connecting an IC chip electrode to an external lead in a semiconductor device, a palladium (Pd)-coated copper wire is used. For example, Japanese Utility Model Application Publication No. Sho 60-160554 proposes “a bonding fine wire for a semiconductor comprising a coating layer made of Pd or a Pd alloy directly or via an intermediate layer on the outer periphery of a core wire made of Cu or a Cu alloy.” Thereafter, as a practical palladium (Pd)-coated copper wire, as disclosed in Japanese Patent Application Publication No. 2004-014884 (Patent Literature 1 which will be described later), “a bonding wire which includes a core and a coating layer formed on the core, in which the core is formed of a material, other than gold, having a micro Vickers hardness of 80 Hv or less and in which the coating layer is formed of a metal whose melting point is 300° C. or more higher than that of the core and whose oxidation resistance is more excellent than that of copper” was developed.
On 47 and the subsequent pages of SEI Technical Review magazine No. 169 published in July, 2006, in a paper titled “Development of Hybrid Bonding Wire” (Non Patent Literature 1 which will be described later) written by Kaimori Shingo et al., “a plating-coated wire which is coated with 0.1 μm of an oxidation-resistant metal and which has the diameter of 25 μm” is introduced. A patent application on the analysis of an interface between the core and the coating layer thereof (Japanese Patent Application Publication No. 2010-272884) is also filed.
In these palladium (Pd)-coated copper wires, as seen in photograph 5 on page 50 in Non Patent Literature 1, on a surface in the vicinity of the root of the molten ball, palladium (Pd) particles are dispersed, and thus a wire loop is stabilized. In the palladium (Pd)-coated copper wires, palladium (Pd) fine particles from a palladium (Pd) drawn layer are dispersed in the molten ball. Hence, it is said that even when in the interface between the molten ball and the aluminum pad, an intermetallic compound of aluminum (Al) and copper (Cu) is produced, the growth rate of the intermetallic compound is lowered as compared with a case of a gold bonding wire.
However, since the conventional palladium (Pd) drawn layer is thinner than the copper (Cu) of the core, most of the palladium (Pd) particles are embedded in the molten ball so as to be formed into an alloy. Hence, although it is known that the palladium (Pd) fine particles function to prevent the formation of an Al—Cu intermetallic compound in the interface with the aluminum pad, the palladium (Pd) fine particles only slightly contribute to the prevention in the interface.
Hence, Japanese Patent Application Publication No. 2013-42105 (Patent Literature 2 which will be described later) proposes an invention of “a bonding wire where on the core of copper and inevitable impurities, a coating layer is formed whose cross-sectional area is 0.1 to 1.0% of the total of the cross-sectional area of the wire and which is formed of Pd” (claim 1 in the publication described above). In a photograph of the surface of the molten ball in FIG. 2a(C) of the publication described above, it is indicated that “Pd (black points) are spread over the FAB (ball b).”
However, the shape of the surface of a core wire or a coated core wire is constantly changed by the wear of a diamond die, and the shape of a cross-sectional surface of the tip of a coated copper wire when the coated copper wire is torn by the second bonding is also constantly changed. Hence, it is extremely difficult to leave the palladium (Pd) fine particles in the thin palladium (Pd) drawn layer on the surface of the molten ball when the FAB is formed. Thus, it is required that the structure of a bonding wire in which even when the FAB is formed 10 or more times for one second, the palladium (Pd) particles can be uniformly dispersed on the molten copper ball and which is suitable for the mass production.